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Related Concept Videos

Chirality at Nitrogen, Phosphorus, and Sulfur02:30

Chirality at Nitrogen, Phosphorus, and Sulfur

Chirality is most prevalent in carbon-based tetrahedral compounds, but this important facet of molecular symmetry extends to sp3-hybridized nitrogen, phosphorus and sulfur centers, including trivalent molecules with lone pairs. Here, the lone pair behaves as a functional group in addition to the other three substituents to form an analogous tetrahedral center that can be chiral.
A consequence of chirality is the need for enantiomeric resolution. While this is theoretically possible for all...
Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule01:10

Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule

In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal (1:1) intensity. When proton A is coupled to two equivalent protons (AX2 spin system), the spin states of each X can be aligned with or against the external field, creating three possible scenarios. This results in a 1:2:1  triplet signal, where the central peak corresponds to the chemical shift of A and is twice as large or intense as the others.
SN2 Reaction: Stereochemistry02:23

SN2 Reaction: Stereochemistry

In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.

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Derivatization of Protein Crystals with I3C using Random Microseed Matrix Screening
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SnB version 2.3: triplet sieve phasing for centrosymmetric structures.

Hongliang Xu1, Alexander B Smith, Nikolaos V Sahinidis

  • 1Hauptman-Woodward Medical Research Institute and Department of Structural Biology, State University of New York at Buffalo, 700 Ellicott Street, Buffalo, NY 14203, USA.

Journal of Applied Crystallography
|May 24, 2008
PubMed
Summary
This summary is machine-generated.

A new version of the SnB direct-methods program now includes the triplet sieve method for phasing centrosymmetric structures. This enhancement can significantly reduce the time needed to solve such structures.

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Area of Science:

  • Crystallography
  • Computational Chemistry

Background:

  • Direct methods are crucial for solving crystal structures.
  • Phasing centrosymmetric structures can be computationally intensive.

Purpose of the Study:

  • To introduce a new version of the SnB direct-methods program.
  • To integrate the triplet sieve method for phasing centrosymmetric structures.

Main Methods:

  • Development of a new version of the SnB program.
  • Incorporation of the triplet sieve phasing procedure.
  • User-transparent implementation of the triplet sieve.

Main Results:

  • The new SnB version successfully integrates the triplet sieve method.
  • The triplet sieve procedure is implemented in a user-transparent manner.
  • Potential for significant reduction in structure solution time for centrosymmetric crystals.

Conclusions:

  • The updated SnB program offers an efficient approach for phasing centrosymmetric structures.
  • The triplet sieve method is a valuable addition to direct-methods software.
  • Further application of this method is expected to accelerate crystallographic structure determination.